It is well accepted fact that the crude oils available to refineries are becoming heavier. Meanwhile, the demand for high value products such as gasoline and middle distillates is increasing. The trend towards heavy feedstock and urgent demand for high quality products coupled with tightening fuel regulations are presenting new challenges for refineries. Among the commercially available options, Fluid Catalytic Cracking (FCC) process is one of the key workhorses for conversion of heavy oils into high-value products.
Designing of FCC catalyst for processing heavy crudes is the key challenges to a catalyst developer for achieving these targets. The current generation FCC catalysts used in cracking process for heavy oils are among the most sophisticated engineered catalysts, having high selectivity towards gasoline range products due to the presence of customized faujasite type zeolites having high acidity, higher hydrothermal stability and discrete pores in the range 6.5 Å to 13.5 Å. Heavy feeds contain high carbon residue, nitrogen, aromatics and contaminants such as nickel, vanadium and other contaminant metals. Processing such feeds while meeting changing product slate, demand catalysts having higher metal tolerance, mesoporous active matrix and small pore zeolite. In a word, the catalyst activity, selectivity, particle size and shape, pore size and distribution, have to be optimized according to the properties of the heavy oils. Further, it is also very difficult to optimize all the said properties within FCC catalyst as this approach can adversely affect the performance of main catalyst. To overcome the above limitations use of additives as separate particles is in practice by the refiner to meet their specific objectives. There are several additive technologies available in the FCC market to obtain yields which meet refinery objectives.
The ability of the feed molecules to reach the active sites of the catalyst is important and for heavy resid molecules, mass transport limitations play an important role. In order to handle the challenges associated with cracking of resid molecules, it is essential to optimize the pore architecture and generation and distribution of weak acid sites in additive formulations.
WO1997012011A1 relates to a process and product for bottom cracking, comprising of aluminosilicate, acid dispersible alumina, phosphate containing ingredient/non-dispersible alumina and C2-C20 alkoxide hydrolysed with water and purified by ion exchange with silicic acid compound.
Cracking catalysts introduced in late nineties are based on dispersible alumina and rare earth exchanged Y zeolites do possess improved hydrothermal stability and higher diesel selectivity (U.S. Pat. Nos. 6,528,447, 6,114,267). However, as the zeolitic material is part of catalyst microsphere, major cracking activity is contributed by these zeolites and as a virtue of higher acid strength, these crack hydrocarbons majorly to gasoline range molecules and partly to LPG range. However, part of heavy hydrocarbon cracking to the boiling range of LCO and heavy naphtha is partly met through use of alumina, silica-alumina matrix or by use of additive referred under WO 97/12011.
U.S. Pat. No. 4,576,709 concerned with upgrading residual oils to gasoline product with a coke selective hydrogen stable faujasite crystalline zeolite catalyst comprising at least 40 weight percent of alumina and rare earth metals. This patent does not disclose use of acid treated fujasite zeolite for bottom up-gradation.
Further, number of patents in the prior art disclose the concept of passivating metal contaminants of nickel, vanadium, copper and iron by the addition of metals and compounds thereof selected from the group of magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, antimony, zinc, cadmium, zirconium, tin, lead and rare earth metals, all of which may or may not contribute to altering product selectivity in a fluid catalytic cracking operation.